This invention relates generally to optical systems, and, more particularly, relates to an apparatus and method for homogenizing radiant energy beams.
The use of lasers and other radiant energy sources has become commonplace in diverse fields, ranging from materials processing to surgery. In many applications, uniform illumination of the target surface, and hence, uniformity of the laser beam intensity profile is of paramount importance. Typical laser generating systems, however, produce laser output beams having spatially non-homogeneous intensity profiles. For example, the radiation may be most intense at the center of the beam and drop off sharply at the periphery, creating a cross-sectional profile resembling a bell-shaped curve similar to a Gaussian intensity profile. Other laser systems produce beams having ring-shaped or star-shaped profiles, which likewise are undesirable in many applications.
Most current work towards improvement of laser beam uniformity has been focused on the configuration of the laser beam generation system. Optical resonator design, excitation electrode design, and improvements in discharge preionization uniformity have increased output beam uniformity significantly. Nonetheless, few commercial lasers can maintain substantially uniform intensity profiles, particularly at high energy outputs Further complicating this problem is the presence of occasional, essentially unpredictable, changes in laser output beam uniformity on a shot-by-shot basis.
Additionally, laser beam non-uniformity can also be introduced during propagation of the beam. In particular, light processing elements, e.g., shutters, blinds, stops, and other similar optical components interposed along the propagation path of the beam can cause substantial distortion of the beam profile.
Several devices have been developed which attempt to improve beam uniformity during propagation between a laser beam source and target. These "homogenizing" devices can be generally categorized as either predictable or unpredictable. In predictable devices, beam homogeneity results from the predictable overlapping of an input beam as it propagates through the device; the homogeneity of the output beam is expressible as a function of the homogeneity of the input beam. In contrast, unpredictable devices cause chaotic overlapping of light rays to such an extent that the spatial coherence of the input beam is destroyed and the energy profile of the output beam becomes a function of the apparatus, i.e., the homogenizer, rather than the input beam.
Predictable light homogenizing apparatuses typically include kaleidoscopes, lens and prism systems, or waveguides having convoluted light transmission paths. (See, for example, Fahlen, U.S. Pat. No. 4,733,944; George et al., United Kingdom Patent Application Number 2,180,363; Telfair, European Patent Application 280,414; Grojean et al., "Production of Flat Top Beam Profiles for High Energy Lasers", 51 Review of Scientific Instruments 375 (March 1980), all incorporated herein by reference). Although useful in some applications, these apparatuses are generally unable to homogenize the large disturbances caused by light processing elements, e.g., shutters, blinds, stops, or other optical components. In addition, beams treated with these devices may exhibit speckle due to the constructive and/or destructive overlapping of the beam during homogenization.
Upredictable devices including, for example, translucent glass, ground glass, and various light scattering polymeric materials, can also be effective light homogenizers; however, such diffusion type devices are unsuitable for many applications. These devices are especially inappropriate for imaging systems due to the unacceptable light losses they produce.
It is an object of the present invention to provide an apparatus and method for homogenizing radiant energy beams regardless of the spatial distribution of the energy in the input beam and in a manner which reduces speckle but avoids the significant losses typically associated with diffusion-type homogenizing devices. Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.